Imba, is often used to indicate an "Uber" character in WoW, but it was originally an abbreviation of "Imbalanced", meaning you where a *very* specialised character... not rounded enough to play "solo" in any means, but therefore the "perfect fit" for a specific role in a "party".
Blizzard did away with "true" Imba, by forcing everyone to complete "solo" quests which required their character to be rounded in order to progress, and charging ridiculous sums to "respec" after each one.
I think this is a "bad" thing, because now, every character comes down to caster or melée. All the other classes and talent builds are pretty irrelevant.
New WoW players don't recognise the difference between "Imba" and "Uber". You used to be able to be "Uber" by being able to solo things 20 levels below you that where supposed to take 5 - 10 player parties, but "Imba" if you could not complete solo runs 40 levels below you, but could pull a 40 man Raid Group of "nubs" through an end game instance, with just 1 other "Imba" character in the Raid.
You always need (used to need) a full Holy Priest or Paly, or Imba Resto Shammy or Drood and either an Imba DPS Mage, Hunter, Shammy, Rogue or Destro Lock or Shadow Priest or an Imba Tank. Which would have to be Defence Warrior, Paly, Shammy or Druid.
A "split" (semi-balanced) Paladin, Shaman or Druid can cover both Tank and Healer slots in a party... but would need an Imba DPS or Healer to back them up... A split Priest, Lock Shaman or Druid could cover DPS and Healing, and a Split Warrior, Shaman or Druid could cover DPS and Tanking... and the "correct" way, is to have an okay Healer, an okay Tank, and an Okay DPS character. (preferably ranged DPS, so no Rogues XD)
DPS Rogue or Affliction Lock was "Imba" for PVP Battle Fields and Arenas... especially the Stun-Lock rogues. (who could keep any character for performing an attack while they add build-ups to a crit and get another paralyses hit preventing them from attacking while they built up again for another crit and stun. It may take them a long time to kill a full Imba protection toon, but there was nothing they could do to prevent the inevitable.
In "duals" the end-game story was, Tank can't take Priest, and Priest can't take Tank. Lock can't take DPS Mage, or Holy Priest. Shadow Priest can't take Lock or DPS Mage. DPS Mage can't take Tank. Hunter can go 50-50 with Mage, Lock or Priest. Shaman, Paladin and Druid can toe-to-toe, depending on spec. So your character class and build is kinda rock-paper-scissors. Your party, or raid had to be balanced. It was most balanced when the members where "imbalanced", to solo, your character alone had to suck for it's level... unless you only like the "rock-paper-scissors" of a Dual / Battle Grounds.
Now every character has to heal, defend and do damage. So it doesn't matter what class you choose. Either for your character, party, or raid. That's just a wasted game feature. But modern WoW has many of those.